KR101210792B1 - Flexible Organic Light-Emitting Diodes And Manufacturing Method Thereof - Google Patents

Abstract

The present invention relates to a FOLED having improved performance and a manufacturing method thereof.
After fabricating the FOLED, the FOLED performance can be improved by lowering the charge injection barrier by trapping ionic impurities in the boundary layer near the electrode by applying an external voltage (E-annealing).
FOLED fabricated according to the present invention exhibits high EL emission and efficiency characteristics.

Description

Flexible Organic Light-Emitting Diodes And Manufacturing Method Thereof

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to flexible organic light-emitting diodes (FOLEDs) and a method of manufacturing the same. More particularly, the present invention relates to a flexible organic light-emitting diode (FOLED) and a method of manufacturing the same. It relates to a manufacturing method for improving the performance of the FOLED and to a FOLED manufactured accordingly.

OLEDs are a very attractive device because they can be used for full-color flat-panel displays and large area illumination. Another big advantage of OLEDs is their flexibility. The key issue for implementing such FOLED is a problem of forming a transparent electrode on a flexible substrate. Indium-tin-oxide (ITO) is widely used for transparent electrodes (anode), but it does not provide stable anode characteristics due to its vulnerability on the flexible substrate.

Thus to replace ITO polyaniline, polypyrrole, polythiophene, and poly (3,4-ethylenedioxythiophene); poly (styrenesulfonate) (poly (3, Extensive research is being conducted on polymer electrode materials such as 4-ethylenedioxythiophene): poly (styrene sulfonate) (PEDOT: PSS).

 Among them, PEDOT: PSS has many advantages such as colorless transparency, high conductivity, easy viscosity changeability, easy film formation, low surface roughness and low price.

Dimethylsulfoxide (DMSO), N-methylpyrrolidone (N-methylpyrrolidone: NMP), N, N-dimethylformamide (N, N-

Adding high boiling polar solvents such as dimethylformamide (DMF) or ethylene glycol (EG) can improve the conductivity and viscosity of PEDOT: PSS. The surface roughness can be improved by adding D-sorbitol. This modified PEDOT: PSS has been used as a transparent positive electrode of OLEDs and organic solar cells. In addition, there have been various reports on the method for patterning polymer electrodes, such as thermal imaging, ultraviolet printing, microcontact printing, lift up and soft embossing. Recently, a simple peel off technique has been reported, which is capable of forming very sharp edges.

However, despite such advances over the last decade, the light emitting performance of FOLEDs using polymer anodes is not yet satisfactory. As a result, research on the manufacturing process that can provide high performance, simple and reproducible ITO-free FOLED has continued.

Accordingly, an object of the present invention is to provide a high performance ITO-free FOLED and a method of manufacturing the same.

The present invention, a flexible substrate;

A polymer anode layer formed on the flexible substrate;

An electroluminescent layer formed on the polymer anode layer;

An electron injection layer formed on the electroluminescent layer; And

It provides a method of manufacturing a flexible organic light emitting device, characterized in that the electric field annealing to an indium tin oxide-free (FTOD) flexible organic light Emitting Diode (FOLED) device comprising a cathode layer formed on the electron injection layer can do.

In addition, the present invention can provide a method for manufacturing a flexible organic light emitting device, characterized in that the electric field annealing is applied by processing a voltage containing 15V to 20V to the FOLED device.

In addition, the present invention, the polymer anode layer may provide a method for manufacturing a flexible organic light emitting device, characterized in that the DMSO doped PEDOT: PSS.

In addition, the present invention can provide a method of manufacturing a flexible organic light emitting device, characterized in that the electric field annealing is performed by applying a voltage pulse.

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According to the present invention, a flexible organic light emitting device having high current efficiency and high luminance can be efficiently manufactured. That is, according to the present invention, a flexible organic light emitting diode having excellent performance can be manufactured with a peak luminance of 6,100 cd / m ² and a maximum current efficiency of 16.4 cd / A.

1A is a cross-sectional view of a flexible organic light emitting device.
1B is a graph showing a voltage pulse for performing E-anneal according to an embodiment of the present invention.
2 is a graph showing current-voltage (JV) and luminance-voltage (LV) of a flexible organic light emitting device manufactured according to the prior art.
3 is a graph showing the current-voltage (JV) characteristics of the flexible organic light emitting device manufactured according to the present invention.
4 is a graph showing luminance-voltage LV of a flexible organic light emitting device manufactured according to the present invention.
5 is a graph showing the current efficiency of the flexible organic light emitting device manufactured according to the present invention.
6 is a graph showing the current density and luminance characteristics of the flexible organic light emitting device according to the DMSO doping concentration.
7 is a graph showing the current efficiency and power efficiency of the flexible organic light emitting device according to the DMSO doping concentration.
8 is a photograph taken to visually recognize the EL light emission of the flexible organic light emitting device manufactured according to the prior art and the present invention.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

1A is a cross-sectional view showing the configuration of a FOLED.

As a substrate 100 of FOLED, a flexible polyethersulfone (PES) film (Glastic PES, manufactured by I-components Co.) is used. The PES film has a thickness of about 0.1 mm, a surface roughness of about 8 nm, and a transmittance of 87% for a 500 nm wavelength.

PEDOT: PSS (CLEVIOS ^™ ) as a polymer anode layer 200 on the PES film substrate. Product of PH 500) can be used. The weight ratio of PEDOT to PSS is 1: 2.5 and the mixture is dispersed in H ₂ O. In addition, the conductivity can be improved by doping DMSO and D-sorbitol in the PEDOT: PSS solution, where the concentration of D-sorbitol is preferably fixed at 0.625 wt%, while the concentration of DMSO is 5.0 wt% to 10.0 wt%. It can be changed up to.

In order to uniformly form the polymer anode layer composed of PEDOT: PSS, a multistep spin-casting process is performed. That is, spin casting is performed for 20 seconds at 500 rpm, 40 seconds at 1000 rpm, and finally at 3500 rpm for 4 minutes. The PEDOT: PSS thin film thus formed exhibits characteristics of 180 nm in thickness, 5.5 nm in surface roughness, 220 Ω / square of sheet resistance, and 500 nm and a transmittance of 88% at a wavelength.

After forming the PEDOT: PSS anode layer, the mixed solution is spin coated at 700 rpm to form a green electroluminescent layer (EL layer) 300. The mixed solution for forming an EL layer is N, N'-diphenyl-N (N, N'-diphenyl-N), N'-bis (3-methylphenyl) -1 (N'-bis (3) which is a hole transport layer. -methylphenyl) -1), 1'biphenyl-4,4'-diamine (TDP) (1'biphenyl-4,4'-

diamine (TPD)),

2- (4-biphenyl) -5- (4-tert-butylphenyl) -1,3,4-oxadiazole (Bu-PBD) (2- (4-biphenylyl) -5- ( 4-tert-butylphenyl)-

1,3,4-oxadiazole (Bu-PBD)),

Tris (2-phenylpyridinato) iridium as green emitting layer

(Ir (ppy) 3)), and

1,2-dichloroethane and chloroform (1,2- as hole transport host materials)

Dichloroethane and chloroform) is composed by mixing PVK in a solvent with a weight ratio of 3: 1.

The thickness of the EL spin coating layer is preferably about 80 nm.

Next, on the EL layer, the electron injection layer 400 is formed at 1 nm with LiF, and subsequently the Al cathode layer 500 is formed at 100 nm. They are formed by thermal evaporation at 0.7 nm / sec under a base pressure of 2 × 10 ⁻⁶ torr.

For the flexible organic light emitting device manufactured as described above (Chroma Meter CS-200 (Kon-

ica Minolta Sensing, INC.)), spectrometer (Ocean's Optics), source-meter (Keithley

2400)) to measure the luminescence properties.

2 is a graph showing the performance of the FOLED manufactured by the above method, in particular, the DMSO concentration of the PEDOT: PSS anode layer is 5 wt%.

Referring to the current-voltage (JV) and luminance-voltage (LV) characteristics of the FOLED according to the related art shown in FIG. 2, the current density rapidly increases after the 3 V applied voltage, but the increase slows down after the applied voltage exceeds 15 V. As a result, the current density is almost saturated and exhibits a curved curve.

In the luminance-voltage characteristic, unlike the increase in current density, the increase in luminance is hardly seen at an applied voltage of less than 12 V, and then rapidly increases while the current density curve is bent due to an increase slowing down to 13 V. This poor device performance poses a serious problem in the application of FOLED with a PEDOT: PSS anode layer. This phenomenon also appears when the doping amount of DMSO is 7.5 to 10.0 wt%.

The above phenomenon can be explained by the following principle.

Most OLEDs using the PEDOT: PSS anode include ionic species such as anionic PSS. The presence of these ionic species greatly affects OLED performance. In particular, the first time bias is applied to the new device fabricated, the ionic species diffuse toward the electrode, the anions towards the anode layer and the cations towards the metal layer. This diffusion causes ion current flow. Thus, the internal electric field is formed in the opposite direction to the electric field applied from the outside. This internal electric field reduces the strength of the effective electric field for carrier injection. This is why FOLED's performance with the PEDOT: PSS anode layer deteriorates.

Accordingly, an embodiment of the present invention proposes a method of improving the performance of the so-called E-annealing process by treating a relatively high electric field (E _ext ) to the sample device. .

E-annealing at high electric field intensities can lead to efficient migration of ionic impurities toward the electrodes, thereby weakening the internal electric field formed in the opposite direction due to the increased distance between ions. Some of these migrated ions may be fixed by a carrier trap site near the electrode. That is, it can be fixed by forming a junction in which ionic impurities are trapped.

Ion species trapped between the EL layer and the electrodes may result in strengthening the electric field across the boundary layer and may lead to changes in surface potential.

The trapped ion cloud thus leads to a reduction of the potential barrier, which in turn improves device performance. In other words, the ion space charge accumulated near the electrode forms a strong electric field, reducing the width or height of the barrier to the injection of charge carriers. Improved injection of electron holes results in a balance of electron holes and can effectively lead to excitation formation and charge recombination in the EL layer, two factors that critically affect OLED high performance. Therefore, an increase in EL light emission with high reproducibility with respect to the E-annealed FOLED can be expected.

In particular, this E-annealing has an effect with a threshold of about 15 V applied voltage. E-annealing at lower applied voltages appears to be unsuccessful in trapping ionic species.

To confirm the effect of E-annealing, device performance was analyzed after the E-annealing treatment. The device was processed by the continuous voltage scanning method to apply voltage applied to the sample device and the characteristics were measured.

FIG. 3 is a graph of continuous measurement of current density of an E-annealed FOLED sample device as a function of applied voltage scanning (primary, secondary and tertiary voltage applied scanning), FIG. 4 is luminance, and FIG. 5 is efficiency. It is shown with Figure 2 in a graph measured.

The E-annealing as described above may also be performed by applying a voltage pulse. In this case, the pulse height (h) is 15 to 20 V and the pulse duration (w), which is the pulse duration, is applied within s for τ time. (See FIG. 1B).

3 is a continuous measurement of the sample element under the same circumstances. The secondary scanning of the applied voltage is performed immediately after the primary scanning.

As can be seen from the graphs, the secondary measurements show a good diode-type current curve without the FOLED of the test subject: i) a large reduction in current density as compared to the first scan and ii) no bending of the J-V curve. As the applied voltage is increased from 0 V, the EL luminance also increases, and the turn-on voltage also decreases (Fig. 4). In the L-V curve, the large increase in luminance even at low current densities means that both holes and electrons are efficiently injected into the EL layer, and their charge injection balance is maintained.

As shown in FIG. 5, it can be seen that the current efficiency is higher in the E-annealed FOLED than in the non-E-annealed device. These results confirm that E-annealing occurred during primary voltage scanning (especially while voltages in the range of 15 to 20 were applied).

That is, the primary scanning is the first voltage scanning after the device fabrication, it can be seen that the E-annealing effect occurs in this process.

After the secondary voltage scanning, a third voltage scanning is performed and device characteristics are measured. In terms of reduced current density, increased brightness and increased efficiency, the results are almost the same as for secondary voltage scanning. The reproducibility for this sample device means that the ions migrated from the primary voltage application remain stable at the contact surface (form an ion trapping junction) and the ions at the interface can increase charge injection into the organic EL layer.

In order to find the optimal conditions for the E-annealed FLOED with PEDOT: PSS anode, the characteristics were observed as shown in FIGS. 6 and 7 while varying the concentration of DMSO added to PEDOT: PSS.

Referring to Fig. 6, it starts to operate near 4 V, which is a relatively low turn-on voltage, and bright EL light emission appears, which means that the EL can efficiently emit light from the E-annealed FOLED. The performance of FOLED with this PEDOT: PSS anode demonstrates the potential superiority of the sample device (without a complete optimization of the operation of the FOLED). In particular, FOLEDs with a DMSO concentration of 7.5 wt% show excellent performance, resulting in brightness of 100 cd / m ² and 1,000 cd / m ² at operating voltage 7.5 and 11.2 V, respectively, and peaks at 6,100 cd / m ² at 26.5 V. Brightness, which is relatively high with ITO-free FOLED in solution processes.

The brightness of this test device is comparable to conventional polymer phosphorescent OLEDs using ITO anodes on glass substrates.

In addition, as shown in Figure 7, the efficiency of the device having a DMSO concentration of 7.5 wt% can be obtained. The current efficiency η _C at 100 cd / m ² reached η _C = 16.2 cd / A, 15.5 cd / A at 1,000 cd / m ² , and η _C = 7.6 cd / A at 6,100 cd / m ² . In addition, the power efficiency η _P was inferred, reaching a maximum of 8.7 lm / W and then slowly decreasing with increasing bias voltage.

These results also indicate that charge injection balance is being achieved for the E-annealed FOLED. EL light emission of the device is CIE color coordinate (Commission Internationale de

L'Eclairage) coordinates x = 0.32, y = 0.61, which was almost independent of current density. The results for this are summarized in Table 1.

DMSO (wt%) characteristic 5.0 7.5 10.0 V _ON [V] 5.2 4.2 3.7 L max [cd / m ² ] 1,484 6,090 4,490 η _P max [lm / W] 1.4 8.7 7.6 η _C max [cd / A] 3.9 16.4 11.7 CIE [x, y]  0.324, 0.611 0.324, 0.611 0.324, 0.611

Next, the EL operation of the E-annealed device was visually observed. FIG. 7 is a photograph showing an operation state of a sample device under a state in which a bias voltage of 12 V is equally applied to a device having 7.5 wt% of DMSO.

From the photograph of FIG. 8, it can be clearly seen that the EL light emission in the active region of the E-annealed FOLED is much brighter than that of the non-E-annealed FOLED. Therefore, it can be seen that device performance is improved due to subsequent ion diffusion trapping by E-annealing.

The rights of the present invention are not limited to the embodiments described above, but are defined by the claims, and those skilled in the art can make various modifications and adaptations within the scope of the claims. It is self-evident.

100: substrate 200: polymer anode layer
300: EL layer 400: electron injection layer
500: cathode layer

Claims (4)

Flexible substrates;
A polymer anode layer formed on the flexible substrate;
An electroluminescent layer formed on the polymer anode layer;
An electron injection layer formed on the electroluminescent layer; And
And a cathode layer formed over the electron injection layer. The method of claim 1, wherein the electric field annealing is performed by applying a voltage including 15 V to 20 V. 5. 3. The method of claim 1, wherein the polymer anode layer is DMSO-doped PEDOT: PSS. 4. The method of manufacturing a flexible organic light emitting device according to claim 1 or 2, wherein the electric field annealing is performed by applying a voltage pulse.

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